Colorized replace output

before:

```
17 (A) lorem ipsum
TODO: Replaced task with:
17 (A) lorem foo ipsum
```

after:

```diff
TODO: Replaced task #17
- (A) lorem ipsum
+ (A) lorem foo ipsum
```
(close approximation)

this is a personal modification and I'm not at all convinced that it belongs into the general distro (not everyone appreciates ANSI escapes, suitable colors depend on terminal configuration, diff-like output might be too developer-centric) - but I figured I'd share it just in case

I suppose the Right Way™ would be to refactor `todo.sh` to encapsulate reporting so a plugin could overwrite it, but that seemed like overkill